The usable signals obtained by MR systems are extremely small. The signals from the systems are derived when nuclei that are aligned by a strong static magnetic field are perturbed by a pulse or pulses comprising a magnetic field rotating at a radio frequency (RF). After the termination of the RF pulse or pulses the perturbed nuclear magnetization precesses at the resonant frequency and tends to return to the unperturbed orientation. During the reverting process, the precessing nuclear magnetizatiion transmits a signal, albeit small, that is detected and used for spectroscopy or imaging; for example. For imaging purposes gradient magnetic fields are also used for locating the source of the signal--that is the location of the precessing and reverting perturbed nuclei in the subject being imaged.
The inherent low amplitude of the signal increases the importance of improving the SNR. Accordingly, scientists in the field are continuously searching for ways and means of improving the SNR of the received signals.
One of the more popular methods of acquiring data for imaging is through the utilization of echos. For example, there is a scan sequence that is known as the "Spin Echo" sequence. In that scan sequence, first a 90 degree RF pulse is applied to the subject simultaneously with the application of a selection gradient pulse. The 90 degree RF pulse perturbs the aligned nuclei in the subject by 90 degress to move the nuclei to a plane (such as X--Y) that is perpendicular to the axis (Z) of the strong static magnetic field. (X,Y,Z are the so called "rotating" Cartesian Coordinates normally used to define MR systems.) After a certain defined time period a second RF pulse is applied, designed to further perturb the already perturbed nuclei 180 degrees in the same plane (X--Y).
The 180 degree RF pulse reverses the planar movement of the perturbed nuclei. The nuclei when first perturbed to the X--Y plane are substantially all aligned; however, they tend to spread or disburse in the X--Y plane and rotate around the Z axis at different speeds and in different directions, i.e. clockwise and counter clockwise because of such things as the inhomogeneity of the strong static magnetic field.
At any rate, certain nuclei rotate faster than other of the nuclei and hence move further from the originally perturbed position in the X--Y plane. When the 180 degree RF pulse is applied, the nuclei are moved 180 degrees in the X--Y plane so that all of the nuclei are moving towards realignment, 180 degrees from the original aligned position in the X--Y plane. Since the nuclei that are furthest from the original aligned position in the X--Y plane are the fastest moving nuclei it is apparent that the nuclei will again substantially all realign. As a matter of fact the realignment occurs at a time equal to the time difference between the application of the 90 degrees pulse and the 180 pulse. The realignment causes a relatively strong signal known as an "echo".
Multiple echoes are obtained by applying multiple 180 degree pulses. Each subsequent echo signal is slightly smaller than the previous echo signal, and the difference in size is a good estimate of the decay or relaxation time T2.
There are other ways of generating the echo signals which are well known to those skilled in the art. The echo signals are used to provide images of a subject. Magnetic gradients as mentioned before are used to locate the source of the echo signals based upon the Larmor equation, which is: EQU f=.gamma.Bo/2 .pi..
Where:
f is the frequency of the received signal; PA1 .gamma. is the gyro magnetic constant based on the nuclear isotope from which the echo is received; PA1 Bo is the magnetic intensity of the field at the point from which the echo is received; and PA1 .pi. is the constant 3.1416. PA1 si is a set of spin echo signals; PA1 i is a number from 1 to N; PA1 PD is the pseudo-density incorporating the nuclear density and all T1 (longitudinal or spin-lattice relaxation time) related items; PA1 TEi is the set of echo times used in obtaining the set Si; and PA1 T2 is the transverse relaxation time.
The echo signals are very small as are all of the free induction (FID) signals. Thus, any means for improving the SNR is worthwhile.
Synthesization has been suggested as a means of improving the SNRr. For example, an article entitled "An analysis of Noise Propogation in Computed T2, Pseudodensity and Synthetic Spin-echo Images" by James R. MacFall et al published in the Medical Physics Journal, Volume 13, at PP 285-292 in May/June 1986 alludes to the fact that under many circumstances a signal synthesized at some echo time can have an SNR superior to that of a directly acquired signal. As described in the article, the synthesis of spin-echo signals assumes a model wherein; EQU Si=PDe.sup.-TEi/T2
where:
The basic reason for synthesizing as described in the article is to obtain the value of T2 and a multiplicity of values of PD after measuring only a few values of Si at different echo times. The synthesis is accomplished by fitting a curve to the measured values (more than two) to provide further values of the pseudo-density and the relaxation time T2. Thus through synthesization, selected points in a matrix are obtained by actual measurements. More measurements are taken than there are unknowns. Therefore, methods such as least square fitting can be used for extrapolating and interpolating the measured points to obtain virtual images at TE values for which no actual measurements were taken.
It is noted in the previously mentioned article that image synthesization might in certain cases, improve the SNR of said virtual images. However, synthesization does not provide a really accurate T2 time. Accordingly although the image synthesization process provides a relatively noise free image, it does not provide a truly correct image.
The reason for the incorrect image is, among other things, that the values of T2 obtained by synthesization fails to take into account the actual slice profile. Accordingly, synthesized images contain errors because among other things of the differences in the actual slice profile and the ideal slice profile.
Accordingly, it is an object of the present invention to make corrections, in general, on the T2 values obtained using MR echo procedures.
It is a further object of the present invention to utilize the method of correcting the T2 time along with image synthesization in order to obtain images with improved SNR and having true T2 values, and then for example to enable replacement of the originally acquired set of images with a new set of images having improved fidelity; i.e., closer to an ideal set.